The present invention relates to catalysts for the homogenous hydrogenation or hydrosilylation of carbonyl compounds. More specifically, the invention relates to processes for the hydrogenation of ketones and aldehydes using organometallic complexes of tungsten (W) and molybdenum (Mo) as catalysts or catalyst precursors. The invention also relates to processes for the hydrosilylation of ketones, aldehydes and esters using the same catalysts or catalyst precursors.
Hydrogenation reactions involve the addition of hydrogen to an organic compound whereby, for example, a ketone can be reduced to an alcohol. Prior art processes have generally required the presence of a heterogeneous catalyst with a solid phase of platinum, rhodium, palladium or nickel along with relatively high hydrogen pressure and elevated temperature.
Other hydrogenation processes currently in use employ inexpensive Mo and W metals to hydrogenate ketones under mild conditions of temperature and pressure. However, a limitation encountered with these processes is the decomposition of the catalysts, due to dissociation of a phosphine ligand.
Hydrosilylation reactions involve the addition of hydrosilane to ketones, aldehydes, or esters to form primarily alkoxysilanes. Prior art hydrosilylation processes have also required rhodium, platinum or palladium complexes as catalysts.
Thus, traditional homogeneous catalysts for hydrogenation or hydrosilylation of ketones or aldehydes use precious metals such as platinum (Pt), rhodium (Rh), iridium (Ir) or ruthenium (Ru), which are expensive and, therefore, frequently uneconomical. In contrast, the catalysts of the present invention, which use either molybdenum (Mo) or tungsten (W), are prepared with less expensive metals, and, therefore, offer economic advantages.
The present invention also relates to recyclable and recoverable homogeneous catalysts including organometallic complexes that can be used in solvent-free catalytic reactions.
Homogeneous catalysts offer many advantages over heterogeneous catalysts, but the pervasive problem of separating the reaction product from the catalyst constitutes a drawback to the utility of many homogeneous systems. There have been attempts in the prior art to facilitate recycling of homogenous catalysts. For example, Zwei, X., et al. in “Reaction—controlled phase—transfer catalysis for propylene epoxidation to propylene oxide, Science, 292, 1139 (2001) exploits a decrease in catalyst solubility when one reagent is consumed. A more general approach utilizing the thermoregulated miscibility of organic and fluorous (fluorinated organic) solvents with catalyst recovery in the fluorous phase is described by Horvath, I. T., in “Fluorous Biphase Chemistry”, Acc. Chem. Res., 31, 641-650, (1998). Other attempts in the prior art used fluorous thermomorphic catalysts, which allow reactions without fluorous solvents and even without solvent at all as described by Wende, M. et al. in “Fluorous Catalysis under Homogeneous Conditions without Fluorous Solvents,” J. Chem. Soc., 125, 5861 (2003).
The principles of green chemistry and green engineering indicate that avoiding the use of solvents is an important way to prevent generation of waste. Furthermore, a solvent-free transformation from pure reagents to pure products potentially yields a dramatic change in the properties of the medium and provides an opportunity for attaining catalyst self-precipitation. Precipitation, in turn, helps to avoid using solvents in the subsequent separation stages, further preventing waste generation. A useful catalyst should stay at least somewhat soluble until the last molecule of the substrate is consumed. Rare instances of such retention of solubility are known among compounds with a low aptitude for crystal lattice formation. Such compounds can furnish a liquid phase—a liquid clathrate—with just a few equivalents of the solvent per equivalent of the otherwise solid component. This behavior is observed among ionic complexes with weakly coordinating counterions, where the charges are delocalized over large molecular fragments, and the crystal packing forces are weakened. However, liquid clathrates have thus far not been used to enable catalysts to remain in a liquid phase in catalytic reactions.
There is, therefore, still a need in the chemical arts for catalysts that can catalyze catalytic reactions in the absence of a solvent. There is also a need for catalysts that can catalyze catalytic reactions in the liquid phase and can be easily separated from products.